How to work the hard bits, because fifteen minutes of repeating a difficult passage at full speed is not much better than playing from the top.
Make the chunk smaller than feels reasonable. Two bars. Sometimes one. If you cannot play it correctly three times in a row, it is still too big.
Slow enough to be perfect. Not slow enough to be nearly right, slow enough that it is genuinely correct every time, including fingering and dynamics. If you are making the mistake at this speed, halve it again.
Hands separately first if they disagree, then together at the speed the harder hand can manage.
Only speed up after three correct repetitions, and by a small step. If it breaks, go back down. This is the whole method and it feels absurdly slow for about a week.
Practise the transition in and out, not only the passage. Most stumbles happen at the joins, and a passage practised in isolation often falls apart when approached from the bar before.
What you are actually doing is refusing to practise mistakes. Playing a passage at full speed with an error in it is rehearsing the error, which is why it has survived a year and why it is always in the same two places.